Fish stock is one of the best, nourishing stocks known to man. It contains thyroid and iodine. Find out why they're important and how to make your own.
Nourishing Traditions notes: Ideally, fish stock is made from the bones of sole or turbot. In Europe, you can buy these fish on the bone. The fish monger skins and filets the fish for you, giving you the filets for your evening meal and the bones for making the stock and final sauce.
Unfortunately, in America sole arrives at the fish market preboned. But snapper, rockfish and other non-oily fish work equally well; and a good fish merchant will save the carcasses for you if you ask him. As he normally throws these carcasses away, he shouldn't charge you for them. Be sure to take the heads as well as the body.
Classic cooking texts advise against using oily fish such as salmon for making broth, probably because highly unsaturated fish oils become rancid during the long cooking process.
Fish Stock - https://wholeintentions.com/fish-heads-in-my-stock-pot/